Exhaust emissions regulations for internal combustion engines have become more stringent over recent years. For example, the regulated emissions of NOx and particulates from diesel-powered internal combustion engines are low enough that, in many cases, the emissions levels cannot be met with improved combustion technologies. Therefore, the use of exhaust after-treatment systems on engines to reduce harmful exhaust emissions is increasing. Typical exhaust after-treatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust after-treatment systems for diesel-powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), a particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust after-treatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through or over the components. The DOC, DPF, and SCR catalyst each include a catalyst bed or substrate that facilitates the corresponding exhaust emissions treatment operation. Generally, the catalyst bed of the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The substrate of the DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the catalyst bed of the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
Generally, the catalyst bed of the SCR catalyst is configured to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant (e.g., diesel exhaust fluid (DEF)) and the SCR catalyst to convert the NOx. In most conventional SCR systems, ammonia is used to reduce NOx. However, due to the undesirability of handling pure ammonia, most systems utilize an alternate compound such as urea, which vaporizes and decomposes to ammonia before entering the SCR catalyst. When just the proper amount and distribution of ammonia is available at the SCR catalyst under the proper conditions, the ammonia reduces NOx in the presence of the SCR catalyst. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from several drawbacks. For example, one known drawback is the inability to effectively provide feedback control of the engine system based on the sensed characteristics of exhaust gas flowing through the SCR system.
Conventional methods for controlling operation of an engine and a reductant doser in an SCR system are based on an open-loop control system. Inputs to the open-loop control system include sensed characteristics of exhaust gas flowing through the system. One or more of the sensed characteristics are compared to a predetermined operating map to obtain an appropriate reductant dosing rate. Typically, the characteristics are sensed at a location upstream of the SCR catalyst of the SCR system. Often, to detect failures or accommodate correction of the map-generated reductant dosing rate, additional characteristics of the exhaust gas sensed at a location downstream of the SCR catatyst can be used. Although a control system employing sensors upstream and downstream of an SCR catalyst provides some benefits, the efficiency and accuracy of the system often suffers with such an arrangement.
Additionally, the design of sensors used in conventional exhaust after-treatment systems for sensing exhaust characteristics often promotes several drawbacks. Typical sensors used in exhaust after-treatment systems are point-measurement devices that sense the concentration of components of the exhaust gas at a single localized point within the exhaust stream. A controller then assigns a component concentration for all the exhaust gas flowing through the system based on the sensed concentration at the localized point. Often, the localized point is at an outer periphery or a center of the exhaust gas stream. In most systems, however, component concentrations within the exhaust gas stream can be poorly spatially distributed. Such poor spatial distribution of components within the exhaust gas can be caused by inadequate mixing of the reductant upstream of the SCR catalyst. Inadequate mixing of reductant upstream of the SCR catalyst can also result in poor distribution of NOx downstream of the SCR catalyst. Component concentration calculations for the entire exhaust gas stream based on readings taken from malidistributed exhaust glow by point-measurement sensors upstream and downstream of the SCR catalyst may be inaccurate. Inaccurate component concentration calculations may lead to measurement errors and potentially negative effects on the efficiency and longevity of an exhaust after-treatment system, particularly an SCR system.
Further, certain probe-type sensors demand a certain exhaust gas flow rate through the probe and past the sensing device for accurate readings. Often, maintaining an adequate exhaust gas flow rate through the probe under a wide range of operating conditions is difficult.